The present invention is generally directed to an implantable medical device, e.g., a cardiac stimulation device, and is particularly directed to an automatic capture/threshold pacing method for use in such a device.
Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation or in a pathologic rapid rhythm and then apply cardioverting or defibrillating electrical energy to the heart to restore and maintain the heart in a normal rhythm. Implantable cardiac stimulation devices may also include the combined functions of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation pulses when they are needed and inhibit the delivery of cardiac stimulation pulses at other times. This inhibition accomplishes two primary functions. Firstly, when the heart is intrinsically stimulated, the patient""s hemodynamics are generally improved. Secondly, inhibiting the delivery of a cardiac stimulation pulse reduces the overall battery current drain and therefore extends the life of the device battery. Extending the battery life, will therefore delay the need to explant and replace the cardiac stimulation device due to an expended battery. Generally, the circuitry used in implantable cardiac stimulation devices has been significantly improved since their introduction such that the major limitation of the battery life is related primarily to the number and amplitude of the stimulation pulses. Accordingly, it is preferable to minimize the number of pulses delivered by using this inhibition function and to minimize the amplitude of the pulses when it is clinically appropriate.
It is well known that the amplitude of a pulse that will reliably stimulate a patient""s heart, i.e., its threshold value, will change over time after implantation and will vary with the patient""s activity level and other physiological factors. To accommodate for these changes, pacemakers may be programmed to deliver a pulse at an amplitude well above an observed threshold value. To avoid wasting battery energy, techniques were developed to automatically adjust the pulse amplitude to accommodate for these long- and short-term physiological changes. For example, an existing device, the Affinity(copyright) DR, Model 5330 L/R Dual-Chamber Pulse Generator, manufactured by the assignee of the present invention, an AutoCapture(trademark) pacing system is provided. The User""s Manual, (copyright)1998 St. Jude Medical, which describes this technique is incorporated herein by reference. In this system, the threshold amplitude level is automatically determined for a predetermined duration level in a threshold search routine and capture is maintained by a capture verification routine. Once the threshold search routine has determined a pulse amplitude that will reliably stimulate, i.e., capture, the patient""s heart, the capture verification routine monitors signals from the patient""s heart to identify pulses that do not stimulate the patient""s heart (indicating a loss-of-capture). Should a loss-of-capture (LOC) occur, the capture verification routine will generate a large amplitude (e.g., 4.5 volt) backup pulse shortly after (typically within 80-100 ms) the original (primary) stimulation pulse. This capture verification occurs on a pulse-by-pulse basis and thus, the patient""s heart will not miss a beat. However, while capture verification ensures the patient""s safety, the delivery of two stimulation pulses (with the second stimulation pulse typically being much larger in amplitude) is potentially wasteful of a limited resource, that is the battery capacity. To avoid this condition, the existing device monitors for the occurrence of two consecutive loss-of-capture events and only increases the amplitude of the primary stimulation pulse when two consecutive loss-of-capture (LOC) events occur, i.e., according to a loss-of-capture criteria. This procedure is repeated, if necessary, until two consecutive pulses are captured, at which time a threshold search routine is triggered. The threshold search routine decreases the primary pulse amplitude until capture is lost on two consecutive pulses and then, in a similar manner to that previously described, increases the pulse amplitude until two consecutive captures are detected. The value of the pulse amplitude when capture thus occurs is defined as the capture threshold. The primary pulse amplitude is then increased by a safety margin value to ensure a primary pulse whose amplitude will exceed the threshold value and thus reliably capture the patient""s heart without the need for frequent backup pulses. In a copending, commonly-assigned U.S. patent application Ser. No. 09/483,908, filed Jan. 18, 2000, entitled xe2x80x9cAn Implantable Cardiac Stimulation Device Having Autocapture/Autothreshold Capability,xe2x80x9d improved loss-of-capture criteria are disclosed which are based upon X out of the last Y beats, where Y is greater than 2 and X is less than Y. U.S. patent application Ser. No. 09/483,908 is incorporated herein by reference in its entirety.
To treat certain heart conditions, e.g., congestive heart failure (CHF), pacing is done on both the right and left sides of the patient""s heart, e.g., at the right and left ventricles. Typically, while stimulation of the right ventricle occurs via a lead implanted in the right ventricular apex, stimulation of the left ventricle is accomplished through a lead implanted within the coronary sinus (CS). It is critical to ensure that stimulation pulses delivered through the left side (CS) lead are captured by the patient""s heart. The energy used for each pulse is a function of the amplitude level (i.e., voltage or current) and the duration of the delivered pulse as shown in the equation:
E=(V2*d)/R
where V is the amplitude of the stimulation pulse, d is its duration and R is the lead impedance.
Tissue in the coronary sinus may have a threshold as high as 6.0 volts and, therefore may require a pulse having at least a 6.0 volt amplitude and a pulse width of 1.0 milliseconds for capture to be obtained. This threshold is significantly higher than what typically exists on the right side of the patient""s heart since the CS stimulation voltage must xe2x80x9creach throughxe2x80x9d the vein tissue before it gets to the active myocardial tissue. Additionally, a larger chronaxie may result from the lead""s larger surface area ring electrode which is typically used for a CS lead. Accordingly, by applying the above equation, a pulse energy level as high as 72 microjoules is determined (assuming a typical lead impedance of 500 ohms). This is a pulse energy level that could rapidly deplete the device battery. By contrast, pacing in the right ventricular apex (RVA), which has an exemplary 1.50 volt threshold and using a 500 microsecond pulse width through a 1000 ohm lead impedance gives rise to a pulse energy level of 1.1 microjoules which is significantly lower than the exemplary pulse energy level determined for the CS lead. With such a large pulse energy difference between the stimulation sites, it is significant that the left side pulse energy level not dictate the right side pulse energy level which would result in almost a 50% waste of power. Furthermore, if the right side energy level dictated the left side energy level, the left side stimulation pulses would not be able to capture the heart. Furthermore, with multiple sites, the chronaxie, rheobase and impedance values are different and may change with time. Accordingly, any solution based upon a relationship between the right and left side stimulation requirements would be time limited. It is not believed that these dual site complications have been addressed in the prior art.
Furthermore, U.S. Pat. No. 5,697,956 to Bornzin, which is incorporated herein by reference, recognized that while the selection of stimulation energy levels was ideally related to the strength-duration curve for the patient""s cardiac tissue, optimal increases in energy levels should also take into account the battery voltage when voltage multipliers (e.g., voltage doublers or triplers) are necessary to achieve a desired stimulation voltage. Accordingly, Bornzin showed an energy curve (see FIG. 7 of Bornzin) that selectively increased either amplitude or duration to increase the stimulation energy level while avoiding use of the voltage multipliers when possible. However, Bornzin did not address these issues in a dual site environment.
Therefore what is needed is a system that can independently and optimally determine the threshold energy for stimulation pulses for the right and left sides of the patient""s heart and therefore minimize battery depletion while ensuring capture at each of the pacing sites.
The present invention provides an improved device and method for automatically determining threshold detection and maintaining capture in a multiple, e.g., dual, site cardiac stimulation device. When multiple site stimulation is used, e.g., for treatment of congestive heart failure or the like, the threshold stimulation energy level at each of the sites will typically be different and, in the case of a lead implanted in the coronary sinus (CS), threshold stimulation energy level may be significantly different, e.g., 50 times greater or more. Accordingly, embodiments of the present invention independently maintain capture for each site and, preferably, independently determine the threshold for each site. In a significant aspect of the present invention, a preferred device periodically determines the chronaxie and rheobase corresponding to a strength-duration curve for each site and sets initial controlled energy levels accordingly. Once each initial controlled energy level is determined, which preferably includes a safety margin, the controlled energy level is increased when a loss-of-capture criteria is met. Furthermore, power expended from the battery is minimized since each site is individually optimized.
Accordingly, a preferred implantable stimulation device is connected to at least two electrodes implanted in a patient""s heart where a first electrode is positioned to stimulate a chamber in the right side of the patient""s heart and the second electrode is positioned to stimulate a corresponding chamber in the left side of the patient""s heart. Periodically, the implantable stimulation device determines strength-duration curves for each side of the patient""s heart. Using the determined strength-duration curves, the stimulation device then determines controlled energy levels for each side of the patient""s heart that are based upon their respective strength-duration curves.
Furthermore, once controlled energy levels have been individually determined for each (i.e., the right and left) side of the patient""s heart, capture is individually monitored and maintained for each side. For example, if a first controlled energy level, which is used to stimulate the right side of the patient""s heart, fails to generate an evoked response, the first controlled energy level is increased. Similarly and independently, a second controlled energy level is used to stimulate the left side of the patient""s heart and the second controlled energy level is increased if an evoked response is absent.
In a further aspect of the present invention, a preferred device takes into account potential losses due to the use of a voltage multiplier (e.g., a voltage doubler or a voltage tripler) and avoids amplitude increases that trigger the voltage multiplier when possible. Accordingly, a preferred device monitors the present battery voltage and the current amplitudes from each site of the cardiac stimulation device and attempts duration increases when such increases avoid triggering the voltage multiplier, e.g., when the other site does not require the voltage multiplier.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.